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How do constitutional courts review laws for conflicts with a constitution?

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Wood-paneled courtroom with judge's bench, American and Nevada flags, wooden chairs and tables, and a mounted animal head above the bench in the Esmeralda County courthouse, Goldfield, Nevada. — Court Judge's bench in a courtroom in Nevada, U.S.

Constitutional courts review laws for conflicts with a constitution through a process known as judicial review, which serves to ensure that legislation conforms to constitutional principles. The methods for this review vary significantly by legal system:1

Centralized vs. Decentralized Review: In many European countries, specialized constitutional courts hold the exclusive authority to declare statutes unconstitutional. Conversely, in the United States, judicial review is "diffused," meaning any court—from trial courts to the Supreme Court—is empowered to determine the constitutionality of a law during the course of deciding a case.234
Concrete vs. Abstract Review: Many constitutional courts can hear both "concrete" and "abstract" disputes. Concrete review occurs when a constitutional question arises within an actual case or controversy. Abstract review allows a court to assess a law's constitutionality without a pending case, sometimes even before the law is promulgated.23
Referral Processes: Questions often reach these courts via referrals from ordinary judges who identify a constitutional issue, or through appeals from parties claiming their rights were violated. In some systems, nonjudicial entities like the executive or parliamentary minorities may also initiate reviews.24

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